This invention relates to methods and apparatus for making rivets and other similarly shaped articles. More particularly, it concerns improvements in such methods and apparatus by which production rates are increased significantly over those of presently available machinery without compromise in other production parameters such as machinery costs, quality control and the like.
In the manufacture of such shaped articles as rivets (both solid and tubular), screws, bolts and the like, a cylindrical blank is first formed usually by severing a length of an elongated rod or wire and then subjected to successive forming or mechanical working operations until the desired final shape is achieved. Machines used for the production of such articles conventionally incorporate a transfer mechanism having a plurality of blank retaining holders and operable to transfer each blank sequentially to successive work stations at which tools are positioned for performing the work operations on each blank. The work stations are spaced to be in alignment with the path of blank holder movement so that each holder and thus each of a plurality of blanks will be presented at each work station. The transfer mechanism stops or dwells at each work station for a time necessary for all operations to be performed before indexing or incrementally moving to the next work station stop. After the first blank is worked to the final shape in this manner and ejected from the holder by which it was transferred through the successive work stations, a blank will be finally shaped and the resulting article ejected for each subsequent dwell period of the transfer mechanism.
The particular working or shaping operation performed at the respective working stations will vary, depending on the article to be formed. However, the formation of headed articles of the referenced type will usually involve cutting and/or loading a blank in a holder at one station, fixing the axial position of the blank in the holder at a second station by application of force to opposite ends of the blank and thus expanding the blank radially into engagement with the holder, shaping the head and body (such as in the formation of tubular rivets) progressively at succeeding stations again by tools which apply force to opposite ends of the blank and ejecting the finished article at a final station. Each of the blank holders, therefore, in addition to functioning as a carrier for transfer of each blank to the successive work stations, functions also as a body die which determines the precise exterior shank configuration of the article, e.g., the diameter and relative length of a rivet shank to be formed for a blank of a preestablished size.
Rivet making machines (a term of art applicable to machinery used to form screws, bolts and other such articles as well as rivets) currently in use for high speed production of shaped metal articles and which optimize the aforementioned principles of operation, may employ as a blank transfer mechanism a rotatable indexing plate positioned axially between reciprocable ram carried head shaping dies and a fixed bed or shoe supporting body punching pins or anvils. The blank holders or body dies are usually formed as open ended bores in the indexing plate or as die inserts fixed in the plate so that the respective ram mounted head shaping dies and bed mounted body pins can cooperate at opposite ends of each blank under ram force while the blanks are retained in position by the plate during each indexed or dwell position thereof. An exemplary disclosure of this type of machinery is found in U.S. Pat. No. 2,786,217 issued Mar. 26, 1957 to Horace L. Johnson.
Although such machines are capable of high speed operation, the maximum production rate in practice is limited by two factors; (1) the duration of transfer mechanism or indexing plate dwell time required to perform the longest single operation and (2) the time required after completion of a working operation or ram stroke to move a blank from one work station to a successive work station at which it must be accurately positioned and fully stopped for the next ram stroke and resulting work operation. It is generally recognized that the operation requiring the longest dwell time is that of cutting and/or loading each indexing plate mounted holder or body die with a blank having a precise length. To achieve the necessary degree of blank length precision at high speeds, a shearing die pair is incorporated in rivet making machines to be driven in synchronism with the ram and plate indexing drive and preferably operated during the time the body die carrying plate is being indexed between stations for loading or during turret dwell periods. Such die cutting mechanisms are disclosed for example, in the aforementioned U.S. Pat. No. 2,786,217 and in U.S. Pat. No. 3,800,348 issued Apr. 2, 1974 to the present inventor. While blank cutting mechanisms of this type satisfy the requirement for blank length precision as well as reduce dwell time required for the blank loading operation, they tend to both complicate the overall machine in terms of number of controlled moving parts and increase the power requirements as a result of added moving part inertia.
The limitation on production speeds as a result of the time required to index the blank transfer mechanism from one station to the other is perhaps more significant than dwell time duration both because blank transfer time is non-productive time in the context of blank working operations and because of the inertial forces which must be accounted for in initiating and stopping index plate movement with precision. Also, blank holder indexing in prior rivet making machines required two directions of index plate movement; namely, axial movement to clear punch tools supported by the fixed bed of the machine and angular or rotational movement between the respective working stations at which the head forming dies and punch tools are aligned. The strength requirements of the index plate to withstand such movement under high operational speeds necessitate a high inertial mass in the plate which, in turn, limits the time required for the indexing operation. In addition, the necessity for axial movement of a relatively heavy index plate during the terminal portion of each heading tool ram stroke contributes to an increased dwell time.
It is apparent, therefore, that although rivet making machines have achieved a highly refined state of development, there is need for improvement particularly for increasing the production rates of such machines while retaining present high standards of production quality.